Autostereoscopic image acquisition method and system

ABSTRACT

The invention relates to a method of acquiring simulated autostereoscopic video images of a scene to be viewed. On the basis of stored data containing three-dimensional information, it implements n simulated cameras, with n&gt;=3, each generating an image of a scene on a given optical axis. The optical axes of the simulated cameras converge at a point situated at the same distance D from the simulated cameras. The scene to be viewed has a nearest point Pp and a farthest point Pe, and the inter-camera distance and the distance Dmin between the simulated cameras and the nearest point Pp are selected in such a manner that for focus varying between the nearest point Pp and the farthest point Pe the angle 2alpha between two adjacent simulated cameras varies between a value not greater than 4.5° for the point Pp and a value not less than 0.2° for the point Pe.

The present invention relates to a method of acquiring simulatedautostereoscopic video images.

BACKGROUND OF THE INVENTION

At present, there is a considerable amount of software on the market forcomputer-aided design (CAD) making it possible to store datacorresponding to three-dimensional information about objects or a sceneto be observed. The software incorporates processing means that enableonly a flat image of the object or the scene to be viewed in perspectiveon a screen at angles of observation that can be selected at will.

Methods of stereoscopic simulation having two view-points have beenproposed, e.g. in European patent applications numbers EP-125 480(HONEYWELL) and EP-172 110 (GIRAVIONS DORAND), however they operate bystoring stereoscopic half-images and are therefore not suitable for usewith the above-mentioned software.

The Applicant has developed an autostereoscopic video system having morethan two elementary images, also referred to as “viewpoints”, typicallypresenting a number n of viewpoints that is equal to 4, and isproprietor of various patents or patent applications relating inparticular to an autostereoscopic video picture-taking device, and inparticular French patent numbers 87 11764 (FR-2 619 664), 93 05381 (FR-2705 007), and 93 05383 (FR-2 704 951).

That autostereoscopic video system makes it possible for an observer,without wearing special spectacles, to see images in relief on a screenthat is fitted with an optical selector such as a lens array, and to doso under good conditions of visual comfort, given that the observer isnot constrained to occupy a precise position for viewing.

The terms “row” and “column” are used respectively to designatehorizontal lines and vertical rows of pixels as seen by an observerwhether standing or sitting, and independently, for example, of thedirection in which a cathode ray display tube is scanned, i.e.horizontally or vertically. For example, on a CRT having verticallyoriented scan lines, such scan “lines” will be referred to as “columns”.

At present, when displaying stored data as defined above, there is notechnical solution making it possible to simulate a display in reliefunder conditions of visual comfort that enable the autostereoscopiceffect to be visible at more than two viewpoints.

Autostereoscopic cameras make use of a lens array, generally an array ofcylindrical lenses, and simulation thereof can lead only to a model thatis extremely complicated requiring large computer power when generatinga complex image having a plurality of interleaved viewpoints that complywith the parameters for autostereoscopy. One such model has beenenvisaged in the article by Pierre ALLIO entitled “Procédé pour la prisede vue vidéo ou la synthèse d'images en relief et la visualisation enrelief” [A method for taking video pictures or for synthesizing imagesin relief and for displaying in relief], published in l'Onde Electrique,Vol. 71, No. 1, pp. 26-32, Jan. 1, 1991, Paris. The model envisaged inthat article implies implementing a specially modified version of asoftware package (Turbo CAD 3D) relying on vector forms being generatedthat are displayed substantially in real time for the purpose ofcomputing images in real time, or for making animated films ofsynthesized images (but not in real time).

The problem posed by the present invention is thus a problem that isdifficult to solve, a priori, and the solutions that can be envisagedwould appear, a priori, to exclude applications outside the laboratory.

The invention is based on the idea that, surprisingly, it is possible tosimulate an auto-stereoscopic camera, a single camera of the typementioned above, by means of a plurality of elementary cameras, i.e. bya picture-taking system that is by definition not autostereoscopic andwhich normally gives rise to large problems of practical implementation,even when there are only two cameras and therefore only two viewpoints,e.g. as in the case of the system known under the name “IMAX 3D” andwhich turns out to be extremely complex when the number of viewpointsexceeds 2.

Another two-camera apparatus is known from IEEE 1988 InternationalConference on Consumer Electronics, Rosemont, Jun. 8, 1988, pp. 178-179,Shinichi Yamaguchi “Stereoscopic video movie camera 3D-CAM”.

That document relates to a “3D-CAM” camera for taking stereoscopicimages from two viewpoints, which images are then displayed inalternation on a screen, viewing being performed through spectacles thatare fitted with liquid crystal shutters controlled so as to transmit oneimage at a time alternately to the left eye and to the right eye.

That document relates to a system for taking images (and not forsimulating them) and it has only two viewpoints, a configuration inwhich problems associated with autostereoscopy do not arise and in whichthe observer needs to have spectacles or else is constrained to remainin a fixed position relative to the screen.

Yet another two-camera apparatus is known from IEEE Transactions onConsumer Electronics, Vol. 37, No. 1, Feb. 1, 1991, pp. 39-43, YasuoTakemura “Stereoscopic video movie camera using 300K pixel IT-CCDsensors”. While viewing, that apparatus requires the wearing ofspectacles fitted with liquid crystal shutters.

European patent application EP-641 132 (MATSUSHITA) relates to atwo-camera apparatus for taking pictures in which the angle between thecameras is determined in such a manner as to make binocular fusion ofthe nearest point possible. That picture-taking apparatus having twoviewpoints (i.e. not a simulation) cannot take into account the specificproblem associated with autostereoscopy having three or more viewpoints.

According to the invention, simulation is made possible by takingappropriate account of the physical, optical, and perception parametersfor an autostereoscopic system having more than two viewpoints.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is thus to provide a method ofacquiring simulated autostereoscopic images that does not requirecomplicated computation.

Another object of the invention is to provide a method making itpossible from a standard database containing synthetic video imagesincluding three-dimensional information to generate autostereoscopicimages enabling viewing in relief on a screen fitted with an array suchas a lens array, without requiring dedicated software to be provided,nor even existing software to be modified.

Another object of the invention is to provide a method making itpossible to simulate animated images in real or quasi-real time.

A simulated camera has an optical center, e.g. a pinhole, and asimulated sensitive surface having a center which is defined as thecross-point of the diagonals of the image that will subsequently beviewed on a screen.

The invention thus provides a method of acquiring simulatedautostereoscopic images, wherein, starting from stored data includingthree-dimensional information about an object or the scene to be viewedon a display screen, it implements n simulated cameras, where n≧3, eachgenerating an image of said scene, each of said images constituting aviewpoint of an autostereoscopic image, the simulated cameras beingequidistant and spaced apart by the same inter-camera distance b whichremains constant while a picture is being taken, the cameras having aconstant field angle, and each of the simulated cameras having an axispassing through its optical center, and a point referred to as thesimulated sharp point, situated substantially at the same distance D′from all of said simulated cameras, wherein the scene to be viewed has anearest point P_(p) and a farthest point P_(e), and wherein theinter-camera distance b and the distance D_(min) between the simulatedset of cameras and the nearest point P_(p) are selected in such a mannerthat for taking said picture and for a sharp point that varies betweenthe nearest point P_(p) and the farthest point P_(e), the angle 2αbetween said axes of two adjacent simulated cameras varies between avalue that is not greater than 4.5° for the point P_(p) and not lessthan 0.20° for the point P_(e).

This method can be implemented in the context of an autostereoscopicsystem having more than two viewpoints, and corresponds to a normaldepth of stereoscopic field, which concept is distinct from theconventional depth of field of an objective lens system and is explainedin greater detail in the description below.

This concept of stereoscopic depth of field is specific toautostereoscopy having more than two viewpoints, and, as explainedbelow, it makes it possible to define conditions for satisfactoryperception in relief.

In a particularly advantageous variant of the method that makes itpossible to extend natural perception of relief to infinity, for a sceneincluding a point P_(e) situated at infinity, the inter-camera distanceb is selected so that for the angle 2α having a value of 0.2°, thesimulated sharp point is situated at a distance D_(max) such that theimage of an object moving from the distance D_(max) to infinity alongthe bisector of said axes of two extreme simulated cameras moves on thedisplay screen through a distance no greater than n² times the pixelpitch, which corresponds to n lenses on the display screen, each lenscovering n pixels.

A particularly advantageous implementation of the method of theinvention concerning the case where the number of viewpoints isincreased by creating additional intermediate viewpoints withoutchanging the stereoscopic base nor the solid display angle, and moreparticularly suitable for use in a high definition context with a numberof viewpoints greater than four, makes it possible to obtain greaterstereoscopic depth of field, which in turn makes it possible, otherthings remaining equal, for the user to have greater latitude ofdisplacement in front of the display screen. In this preferredimplementation, starting from stored data including three-dimensionalinformation about the scene to be viewed on a display screen, the methodimplements n simulated cameras, where n>4, each generating an image ofsaid scene, the simulated cameras being equidistant and spaced apart bythe same inter-camera distance b which remains constant while a pictureis being taken, the simulated cameras having a constant field angle,each of the simulated cameras having an axis passing through its opticalcenter, and a point referred to as the simulated sharp point, situatedsubstantially at the same distance D′ from all of said simulatedcameras, the scene to be viewed has a nearest point P_(p) and a farthestpoint P_(e), and the inter-camera distance b and the distance D_(min)between the simulated set of cameras and the nearest point P_(p) areselected in such a manner that for taking said picture and for a sharppoint that varies between the nearest point P_(p) and the farthest pointP_(e), the angle 2α between said axes of two adjacent simulated camerasvaries between a value that is not greater than 18°/n for the pointP_(p) and not less than 0.8/n for the point P_(e).

In a particularly advantageous variant enabling natural perception ofrelief to extend to infinity, for a scene including a point P_(e)situated at infinity, the inter-camera distance b is selected so thatfor the angle 2α having a value of 0.8°/n, the simulated sharp point issituated at a distance D_(max) such that the image of an object movingfrom the distance D_(max) to infinity along the bisector of said axes oftwo extreme simulated cameras moves on the display screen through adistance no greater than n² times the pixel pitch, thus giving rise bothto a sharp image and to a sensation of continuous movement between theviewpoints all the way to infinity.

The method of the invention thus makes it possible to perform simulationin particularly simple manner, which is paradoxical, given that it iswell known that stereoscopic systems having a plurality of cameras areparticularly complicated and difficult to implement, even when they arerestricted to two viewpoints and two cameras.

The said axes may be the optical axes of the simulated cameras.

In a preferred implementation, the simulated cameras have theirsimulated sensitive surfaces mutually parallel and disposedsubstantially in a common plane.

In a particularly advantageous implementation, the simulated cameras areof the pinhole type.

When viewing is by way of a television screen having a lens array placedin front of it, each elementary image can be directly obtained in theanamorphosed format corresponding to its degree of autostereoscopy byallocating vertical resolution to each image point of the simulatedcamera equal to the vertical resolution of the video image, andhorizontal resolution equal to the horizontal resolution of the videoimage divided by the number n of viewpoints. Thereafter, it suffices tointerleave the elementary images obtained in order to obtain a simulatedautostereoscopic image that can be viewed on a television screen.

In the invention, it is also possible to obtain an effect of apparentlychanging the size of an object or of a scene without disturbing itsshape from focusing at said distance D′ by moving the simulated camerasrelative to said object without changing either the sharp point or theangle 2α, and by modifying the value of b in proportion to themodification in the focusing distance.

In a preferred implementation, the image includes stereoscopic pairshaving a given stereoscopic base B, the viewpoints of said stereoscopicpair being separated by m intermediate viewpoints, where m is aninteger >1. It may include a step of viewing under conditions where, atthe ideal, “solid color” distance, an observer sees one of saidstereoscopic pairs having viewpoints that are separated by m elementaryviewpoints.

The invention also provides an autostereoscopic video system comprising:

apparatus for acquiring simulated autostereoscopic video images of ascene to be viewed, the apparatus comprising a database containingstored data including three-dimensional information about an object orthe scene to be viewed on a display screen, apparatus for generating nsimulated cameras, where n≧3, each generating an image of said scene,the simulated cameras being equidistant and spaced apart by a commoninter-camera distance b which remains constant while taking pictureswith a constant field angle, and each having an axis passing through itsoptical center and through a “simulated sharp point” situatedsubstantially at the same distance D′ from all of said simulatedcameras, the scene to be viewed having a nearest point P_(p) and afarthest point P_(e), said apparatus for generating n simulated camerasbeing set up so that the inter-camera distance b and the distanceD_(min) between the set of simulated cameras and the nearest point P_(p)satisfies the condition whereby, for said picture and for focusingvarying between the nearest point P_(p) and the farthest point P_(e),the angle 2α between said axes of two adjacent simulated cameras variesbetween a value not greater than 4.5° for the point P_(p) and a valuenot less than 0.2° for the point P_(e); and

display apparatus in which an observer at the ideal, “solid color”distance sees a stereoscopic pair comprising two viewpoints separated bym intermediate viewpoints where m is greater than or equal to 1.

In the system the apparatus for generating n simulated camera may be setup so that, for a scene having a point P_(e) situated at infinity, theinter-camera distance b is such that for the angle 2α having a valueequal to 0.2°, the simulated sharp point P is situated at a distanceD_(max) such that the image of an object moving from the distanceD_(max) to infinity along the bisector of said axes of the two extremesimulated cameras moves on the display screen through a distance nogreater than n² times the pixel pitch.

The invention also provides a system enabling the number of viewpointsto be increased by creating additional intermediate viewpoints, thesystem comprising:

apparatus for acquiring simulated stereoscopic video images of an objector of a scene to be viewed, the apparatus comprising a databasecontaining stored data including three-dimensional information about theobject or the scene to be viewed on a display screen, apparatus forgenerating n simulated cameras, where n>4, each generating an image ofsaid scene, the simulated cameras being equidistant and spaced apart bya common inter-camera distance b which remains constant while takingpictures, and each of the simulated cameras has an axis passing throughits optical center and a “simulated sharp point” situated substantiallyat the same distance D′ from all of said simulated cameras, the scene tobe viewed having a nearest point P_(p) and a farthest point P_(e), andsaid apparatus for generating n simulated cameras being set up so thatthe inter-camera distance b and the distance D_(min) between the set ofsimulated cameras and the nearest point P_(p) satisfies the conditionwhereby, for said picture and for focusing varying between the nearestpoint P_(p) and the first point P_(e), the angle 2α between said axes oftwo adjacent simulated cameras varies between a value not greater than18°/n for the point P_(p) and a value not less than 0.8°/n for the pointP_(e); and

display apparatus in which an observer at the ideal, “solid color”distance sees a stereoscopic pair comprising two viewpoints separated bym intermediate viewpoints where m is greater than or equal to 1.

In the system the apparatus for generating n simulated camera may be setup so that, for a scene having a point P_(e) situated at infinity, theinter-camera distance b is such that for the angle 2α having a valueequal to 0.8°/n, the simulated sharp point is situated at a distanceD_(max) such that the image of an object moving from the distanceD_(max) to infinity along the bisector of said axes of the two extremesimulated cameras (C₁, C₄) moves on the display screen through adistance no greater than n² times the pixel pitch.

The said axes may be the optical axes of the simulated cameras.

In a preferred embodiment, the simulated cameras have their simulatedsensitive surfaces mutually parallel and disposed substantially in acommon plane.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear moreclearly on reading the following description, with reference to theaccompanying drawings, in which:

FIG. 1 is a diagram showing how the invention is implemented by means ofsimulated cameras, e.g. pinhole cameras;

FIG. 2a is a detail of FIG. 1, showing the case of pinhole cameras, FIG.2b being an illustration of the simulated screen of a pinhole camera ofthe invention;

FIGS. 3a and 3 b show two cases of an autostereoscopic image being builtback up from elementary images supplied by the simulated cameras of thepreceding figures;

FIGS. 4a to 4 d show four advantageous variants of the invention;

FIG. 5 shows the condition for uniform autostereoscopy in accordancewith the invention; and

FIG. 6 shows a preferred embodiment of the invention.

MORE DETAILED DESCRIPTION

To define the principles for making an autostereoscopic image fromsynthetic images, it is necessary to simulate by computation a compleximage that could be obtained using a camera as described in theApplicant's French patents No. 87 11764 (FR-2 619 664); No. 93 05381(FR-A 2 705 007); and No. 93 05383 (FR-2 704 951).

According to the invention, in order to create this complex video image,it is not necessary to introduce a lens array if the de-interleavedimage is considered as being obtained by the treatment described in theApplicant's French patent No. 93 03580 (FR-2 705 006). The imageobtained in this way by the so-called “n image” mode can, forcomputation, be implemented by simulated cameras, providing they arevirtually positioned in a particular manner as described below. Thesimulated cameras are advantageously equivalent to pinhole cameras. Theresolution of the sensitive surface can be anisotropic such that foreach image point or pixel, the resolution considered in the verticaldirection is equal to the vertical resolution of the final image, whilethe horizontal resolution is equal to the horizontal resolution of thefinal image divided by n (the number of viewpoints). This makes displayon a television screen possible. However, for a back-projection displayusing n projectors, the resolution remains isotropic.

With reference to FIGS. 1 and 2a, the n main optical axes A₁ to A₄, i.e.the straight lines passing through the midpoints P₀ of the simulatedsensitive surfaces (E₁, . . . , E₄) or the equivalent, and perpendicularto said surfaces (E₁, . . . , E₄), pass through the pinholes (O₁, . . ., O₄) and intersect at a single point P at a focusing distance D of thesimulated autostereoscopic camera C. The angles (2α) formed from saidsingle point and the n optical axes of the simulated system, taken inpairs, lie between two limit values which are defined as a function ofthe particular observation conditions associated with n-viewpoint TVautostereoscopy.

To define the conditions at the limit and the method of simulatingand/or implementing said autostereoscopic images, it is necessary tostart from the above-defined autostereoscopic image and to analyze itscharacteristics.

In the autostereoscopic system envisaged in the invention, at least oneadditional viewpoint is created between the two viewpoints considered asbeing those that would be seen by the two eyes of an observer at theideal, or “solid color”, distance as defined below. The coherencebetween the viewpoints must be sufficiently close to that obtained bythe above-mentioned autostereoscopic camera. It is also advantageous tocreate at least one additional viewpoint laterally to allow the head tomove horizontally parallel to the plane of the screen, giving a total offour or more viewpoints. The set of viewpoints makes it possible tocreate an overall volume in which the observer can move and observe thescreen (in particular on an axis perpendicular to the screen) withoutlosing the perception of relief. It is this property which characterizesautostereoscopy compared with systems that use stereoscopic viewpointsin pairs only, without any intermediate viewpoints as defined above, andthat constrain the observer to keep in a fixed position relative to thescreen if viewing is performed without spectacles.

In a real case using a camera corresponding to the above-mentionedpatents of the Applicant, the system has been built to enable4-viewpoint images to be taken and viewed simultaneously withoutspectacles, using a single standard PAL CCIR camera at 50 Hz, a singleobjective lens system, and a standard video screen capable of displayingimages in progressive mode (i.e. non-interlaced) with the axes of thelenses in the array being placed parallel to the rows of the image.

For viewing in an implementation corresponding to a preferredimplementation of the present invention, the geometry of the image isadjusted initially to put it into correspondence with the lens arrayplaced in front of the video screen so that the pitch of the lens arrayis substantially equal to the pitch of packets of four pixels, where “4”corresponds to the number of viewpoints selected for taking images inthis particular case, so as to enable an eye of an observer at theselected observation distance to observe only one pixel modulo fourthrough the lens array, which selected distance corresponds to theideal, solid color distance.

Supposing that the observer closes one eye and moves the head parallelto the plane of the screen while keeping at a constant distance from theimage, then that eye will see all of the viewpoints numbers 1, 2, 3, and4 go past in succession. After seeing all of the viewpoints, the eyewill see viewpoint No. 1 again, and thereafter, with continuingmovement, the eye will see the following viewpoint numbers: 1, 2, 3, 4;1, 2, 3, 4; 1, 2, 3, 4; 1, 2, 3, 4; 1, 2, 3, 4; etc. In other words,this is a modulo system in which the same viewpoints are seen severaltimes over when the observer moves parallel to the screen. Each packetof four viewpoints corresponds to a viewing angle which depends on theimage area occupied by four pixels on the screen and on the focal lengthof the array used for display purposes. The pitch of the lens arrayplaced in front of the screen must be slightly smaller than that of thepackets of four displayed pixels.

If respective references (a), (b), (c), and (d) are used for the sets ofpixels in the first, second, third, and fourth of four successivecolumns of pixels in the video image, viewpoint No. 1 comprises allpixels (a) in the image, No. 2 all pixels (b), No. 3 all pixels (c), andNo. 4 all pixels (d). At the ideal distance, the observer will see allof the pixels (a) with one eye and all of the pixels (b) or (c) or (d)with the other eye. Practice shows that it is desirable to present thefollowing pairs [(a) and (c)] or [(b) and (d)] at the nominal displaydistance which is the ideal, solid color distance l. If the spectatormoves towards the screen, the edges of the solid angles move toward eachother given that the inter-pupil spacing between the eyes is constant,so the spectator sees the pair (a) and (d). If the spectator moves awayfrom the screen, the edges of the solid angle (lobe) move apart, so thespectator sees pairs [(a) and (b)] or [(b) and (c)] or [(c) and (d)]. Inreality, because the images have been taken using the autostereoscopicsystem constituted by a single camera having a lens array and a singleobjective lens system, it is possible for the observer to move away fromthe ideal, solid color distance (initially selected distance) because ofthe exceptional coherence of the four viewpoints. These four viewpointsare as coherent as the multitude of images contained simultaneously in abright image with low depth of field taken using a wide open objectivelens system. This property also exists in the case simulated using thepreferred implementation of the invention.

At the nominal observation distance (which is the ideal, solid colordistance), the observer sees (for n=4) a stereoscopic pair formed inaccordance with the invention by the first I₁ and third I₃ viewpoints orby the second I₂ and fourth I₄ viewpoints. This choice of parameterswhereby a stereoscopic pair is formed not by two adjacent viewpoints butby two viewpoints having an intermediate viewpoint between them (or evenm intermediate viewpoints where m≧1, in which case the elementarystereoscopic base between two adjacent viewpoints is equal to B/(m+1), Bbeing the selected stereoscopic base) makes it possible for an observernot wearing special spectacles to have available a viewing volume inwhich the observer can move both parallel to the display screen andperpendicularly thereto, and characterizes uniform autostereoscopywithin the meaning of the present application. B can be selected to beless than, equal to, or greater than the inter-pupil distance E (65 mm)of an observer.

As a result, and referring to the above example, an observer can movetowards or away from the screen from the ideal, solid color distance orcan indeed move laterally without losing stereoscopic vision.

Once the image is on the screen, if the spectator moves towards thedisplay screen (which may be a projecting screen (e.g. a TV screen) or aback-projection screen) from the nominal observation distance (or idealsolid color distance), then the stereoscopic base as perceived growslarger, and if the spectator moves away from the screen, then it becomessmaller, and the overall sensation is constant because this variation inthe stereoscopic base compensates exactly for variation in the sensationof depth associated with changes in vergence forces, i.e. the muscularforces implemented to cause the two rectinal images to overlap, therebyensuring the stereoscopic fusion necessary for perception in relief,which necessarily accompany movements in the direction perpendicular tothe screen.

When the “solid color” has been adjusted so that the observer seesviewpoints [(I₁) and (I₃)] or [(I₂) and (I₄)], and the observer movesfar enough towards the display screen, then the user sees viewpoints(I₁) and (I₄) and can no longer move parallel to the screen, as in areal situation when looking from close up. When the observer moves awayfrom the screen, then one of the following pairs of viewpoints will beseen [(I₁) and (I₂)] or [(I₂) and (I₃)] or [(I₃) and (I₄)] and the usercan move over a considerable distance, thus giving the observer theoption of moving within a volume.

The same applies, with increased comfort, for cases where m is selectedto be greater than 1.

FIG. 5 shows the condition for “solid color” when viewing a televisionscreen. The image has interleaved columns of pixels. Each lens in thearray RV of pitch P_(r) corresponds to n columns of pixels on thescreen. The centers C of the lenses are spaced apart by ΔD₀ in the pixelplane on the screen. The pixel pitch is p′_(p). P′=np′_(p). At the“solid color” distance D₀, the following applies, conventionally:$\frac{P^{\prime}}{P_{r}} = \frac{D_{0} + {\Delta \quad D_{0}}}{D_{0}}$

The condition for uniform autostereoscopy (observing two viewpointsseparated by m intermediate viewpoints when at the “solid color”distance) is: ΔD₀=(m+1)p′_(p)D₀/E, where m is greater than or equal to1.

Uniform autostereoscopy is thus a “modulo” system enabling an observerto move both parallel and perpendicular to a screen while conserving theperception of relief.

It is essentially different from the technique described in the articlepublished in IEEE Transactions Electron Devices, Vol. 22, No. 9,September 1975, New York, by Y. Tsunoda entitled “Three-dimensionalcolor display by projection-type composition holography”, at pp.784-785. That document relates to a stereoscopic display device usingcomposite holography, and comprising N viewpoints.

Pictures are taken therein by angular scanning in N directions spacedapart by equal angles θ₀=θ/(N−1) so as to enlarge the total displayangle which thus becomes equal to θ=40°, e.g. with θ₀=4° and N=11viewpoints. The stereoscopic base between two adjacent viewpoints isequal to the inter-pupil distance. It is not a “modulo” system in whichthe same viewpoints are seen several times over when the observer movesparallel to the screen. In addition, there is no question of causing theangles to vary as a function of focusing distance, given that θ₀ isselected to be equal to the aperture angle (±2°) of the main lobe of thelenses in the display lens array. The observer must also remain at adistance from the screen that is determined in advance.

For a stereoscopic system having two viewpoints, or a stereoscopicsystem having stereoscopic images in pairs without intermediateviewpoints, it is recalled that observing without special spectaclesrequires the observer to occupy a fixed position that is determined veryprecisely, and an observer can be allowed to move only if wearingspecial separating spectacles (e.g. using colored glass orpolarization), or wearing spectacles having liquid crystals that arecontrolled by infrared and that shut off the right eye and the left eyein alternation as in the above-mentioned IMAX 3D method.

To obtain synthetic images or recombined real images in a preferredimplementation of the invention, it is necessary to reproduce thecharacteristics that are specific to the autostereoscopic camera if thebasic qualities of uniform autostereoscopy as defined above are to beenjoyed, i.e. viewing a screen without wearing special spectacles whilestill leaving the observer free to move in a considerable volume.

With the limiting values defined below, these conditions constitute theprinciples that are necessary for acquiring simulated autostereoscopicimages that can be viewed on a video screen in the preferredimplementation of the invention.

Four main parameters need to be taken into consideration for synthesiscomputation or for recombining flat stereoscopic images in pairs toobtain an autostereoscopic image:

1) the total available stereoscopic base, and more particularly theeffective stereoscopic base B/(m+1) between two adjacent viewpoints;

2) the value of the angle 2α between the optical axes of two adjacentsimulated pinhole cameras; these axes converge on a point ofintersection situated at the distance of the simulated focus plane; thisfocus plane corresponds to the plane of the display screen if nohorizontal translation is performed between the viewpoints on the image;

3) the horizontal resolution of the complete image, and the horizontalresolution of each viewpoint; and

4) the field angle of the simulated picture-taking system.

According to the invention, the pictures are taken simultaneously with nsimulated cameras, with two adjacent cameras being spaced apart by adistance that is smaller than the distance between the eyes of anobserver for an observation distance and a scene volume corresponding toviewing the displayed scene without enlargement or reduction.

At least three viewpoints are needed, with the number four being thepreferred value for standard video definition (PAL, SECAM, or NTSC). InFIGS. 1 and 2a, four viewpoints are considered, which viewpoints aresimulated by four pinhole cameras referenced C₁ to C₄, each having arespective pinhole O₁ to O₄, and a sensitive surface or screen E₁ to E₄each having a center P₀. The screen E₁, . . . , E₄ of each simulatedcamera is situated at a distance f, referred to as the focal length,from the pinhole O₁, . . . , O₄, and it is of a width e. Such asimulation can be performed, for example, using the TURBO CAD 3Dsoftware package sold by HANDSHAKE, e.g. by generating each of thesimulated cameras in succession.

When considering a system that is set up in such a manner that thesimulated focal length D₀ and the real viewing distance in front of thescreen (distance at which solid color is achieved) are equal and suchthat the image field and the size on the display screen ensure that anobject which is 10 cm high in the focus plane is 10 cm high on thescreen, which corresponds to linear magnification equal to 1, then, inthe preferred embodiment (uniform autostereoscopy) and for m=1, it isnecessary for the simulated stereoscopic base between two viewpointsthat are visible simultaneously [(a) and (c)] or [(b) and (d)] at thisobservation distance to be as close as possible to the distance betweenthe pupils of the observer, or in other words for the distance betweentwo adjacent simulated cameras to be as close as possible to half theinter-pupil distance of the observer. Under such circumstances, theperception of relief is also the same as for the real scene.

Consequently, once the image is on the screen, if the spectator movestowards the display screen from the ideal, solid color distance, thenthe stereoscopic base actually perceived grows, and if the observermoves away from the screen, then the stereoscopic base shrinks and theoverall sensation is constant because this variation in stereoscopicbase compensates exactly for the variation in the sensation of depththat is associated with the change in vergence forces, i.e. the muscularforces applied to bring the two rectinal images into register so as toobtain the stereoscopic fusion necessary for perception in relief, whichvariation necessarily accompanies movement in the directionperpendicular to the screen.

When the “solid color” is set up so that the observer sees the points ofview [(a) and (c)] or [(b) and (d)], and the observer moves close enoughto the display screen, then the observer sees viewpoints [(a) and (d)]and can no longer move parallel to the screen, as in a real case whenone is looking from close up. On going away from the screen, theobserver sees viewpoints [(a) and (b)] or [(b) and (c)] or [(c) and (d)]and is free to move considerably, thus making it possible for theobserver to move in a volume.

The notion of magnification is important since it makes it possible todetermine the stereoscopic base required for a given angle 2α betweentwo adjacent simulated cameras as a function of the volume of the scenesto be built up or reconstructed, even though the display screenparameters do not change. Surprisingly, it is possible in simple mannerto simulate scenes while preserving all of the parameters with theexception of the size ratio between the computed or reconstituted sceneand the scene perceived on the screen. For this purpose, it is necessaryto adopt an inter-camera distance that keeps the angle 2α within certainlimits. The volume of the scene and the observation distance are changedautomatically. This leads to “macro” and even “micro” systems beingsimulated directly, and in the other direction to “hyper” systems alsobeing simulated, while nevertheless conserving realistic perception ofrelief, the three dimensions remaining in a ratio such as to avoidexceeding the capacity of the observer to compensate perception.

If a given system is now considered, and if the simulated focusingdistance is changed, for constant stereoscopic base, then it is theangle 2α between adjacent viewpoints that changes. For constant imagefield, and depending on the available horizontal monocular resolution,it is possible to distinguish objects that are spaced apart from oneanother in the depth direction with a certain amount of resolution thatcorresponds to the distance to be travelled to give rise to anobservable change (or disparity) from one viewpoint to another. Giventhat the observer does not change distance from the screen beyond thepossibilities described above, the disparity difference betweenviewpoints (greater disparity if the simulated focusing distance is lessthan the distance from the observer to the screen or smaller disparityif the simulated focusing distance is greater than said distance) isperceived as a deformation of the impression of depth compared with thereality that is simulated by the spectator having the eyes located atthe pupil of the objective lens system of the simulated autostereoscopiccamera. If the focus is too short, the observer perceives exaggerateddepth. If the focus is too long, the observer perceives shrunken depth.In both cases there is lack of coherence in the image between its twodimensions (corresponding to a flat image) and the third dimension ordepth, and such incoherence can exceed the capacity of the observer tocompensate perception. This phenomenon must be taken into account whendetermining the limit angles 2α that must not be exceeded in order topreserve a “natural” impression. At greater and smaller values, it isnecessary to modify the stereoscopic base to bring the value of theangle 2α back to within its limits as a function of the simulatedfocusing distances.

This leads to creating changes of scale in computed or reconstitutedscenes, and thus to matching the linear magnification of the image withdepth perception. The horizontal resolution usable for each viewpointdefines the size of depth segments that can be analyzed and observed inthe image, and consequently the number of segments between the minimumand maximum focusing distances corresponding to the limits on the angle2α, while not forgetting that the stereoscopic depth of field observableon the screen depends on the total number of viewpoints used, and on thepitch of the optical selector (lens array) on the screen.

If it were possible to simulate taking a picture with a continuous totalpupil, with the objects being completely sharp and/or moving relative toone another in continuous manner when the observer moves the headparallel to the plane of the screen, the objects shown on the screencould appear to move on the screen between the limit viewpoints (a) and(d) through a horizontal distance equal only to the number n ofviewpoints multiplied by the pitch of optical selector, i.e. by n²pixels.

It is this maximum displacement that defines the notion of stereoscopicdepth of field.

If the simulation is performed with a pinhole camera, objects appear tomove discontinuously, however the value of the stereoscopic depth offield remains the same as in the case of a continuous pupil.

With a real continuous pupil, objects therefore appear out-of-focus assoon as they have moved through more than n lenses with n viewpoints,and n pixels per lens, i.e. a displacement of more than n² pixels. Thediscrete nature of a CCD sensor gives rise to a compromise betweengenuine continuity and the discreteness associated with pinhole cameras.

The observed depth of field is defined as the distance between thenearest point P_(p) and the farthest point P_(e) of the scene to beviewed. The viewed depth of field can be increased only by simulatingpoint-sized pupils or a non-continuous overall pupil (which iseffectively what happens with pinhole cameras) with objects appearing tomove in discrete jumps if they lie beyond a certain apparent distance onthe display screen (⅓ in front and ⅔ behind the display distance), withthe size of the jumps increasing with increasing distance from thescreen, and with the objects remaining in focus.

When a stereoscopic depth of field is used that causes the objects onwhich focusing is not performed to move in discrete jumps, the initialviewing conditions deteriorate when the discrete jumps become too large,to such an extent as to make observation possible only in the region ofspace in front of the screen where solid color is perfect. Objects shownat a distance from the screen appear to be made up of broken fragmentsthat have been stuck together, having either too much or too littlematerial. This happens in particular if the picture is made underconditions that exaggerate depth (“hyperstereoscopy”). Drawbacks thenaccumulate and, disturbance to the perception of relief is associatedwith discomfort due to the observation space being greatly limited. Itis not desirable to make use of too great a depth of field for the sceneto be displayed if, as is the case for television, available displayresolution is limited. It is very tiring to observe objects that are farfrom the screen because our capacity to dissociate binocular vergenceand monocular accommodation is limited. The object will be seen out offocus and in diplopia (failure of binocular fusion).

With reference to FIG. 2, the following applies:$p = {f\quad \frac{\left( {d - D} \right) \cdot {\cos \left( {{\pi/2} - \alpha} \right)}}{{{d \cdot \sin}\quad \left( {{\pi/2} - \alpha} \right)} + {\left\lbrack {D \cdot {\sin^{2}(\alpha)}} \right\rbrack/{\cos (\alpha)}}}}$${{and}\quad f} = \frac{}{2 \cdot {\tan \left( {\theta/2} \right)}}$${i.e.\quad p} = {f\frac{{\left( {d - D} \right) \cdot \tan}\quad \alpha}{d + {{D \cdot \tan^{2}}\alpha}}}$

where:

p=the projection distance relative to the center P₀ of the projectionscreen;

f=the distance between the pinhole from the plane of the projectionscreen E, referred to as the focal length of a simulated camera;

D=the distance at which the aiming axes of the cameras converge on thesharp point P (D=OP);

d=the distance from the observed point to point O;

α=half the angle between two cameras (expressed in radians);

e=the width of the projection screen; and

θ=the full aperture angle of the field of observation of a simulatedcamera, i.e. its field angle.

The data for computing and evaluating the qualities of systemsimplemented are as follows:

the focal length f and the image width e which determine the fieldangle;

the horizontal resolution and the image width e which determine pixelsize;

the focal length f and the aperture which determine the useful diameterof the pupil and the total stereoscopic base;

the total stereoscopic base and the selected number n of viewpointswhich determine the useful base b between two adjacent simulatedcameras, and thus between two adjacent viewpoints;

the stereoscopic base b=O₁O₂=O₂O₃=O₃O₄ between pairs of adjacentviewpoints, and the focusing distance OP (or the distance of theobserved point corresponding simultaneously on the “monocular”viewpoints”) which determine the angle 2α formed by the two optical axesof two adjacent cameras corresponding to two adjacent viewpoints; and

the horizontal resolution, the focusing distance OP, and thestereoscopic base between two contiguous viewpoints which determine thestereoscopic depth of field while viewing.

The perception of relief is determined by the ratio that exists betweenthe distance b and the variation in depth ΔD=d−D. This ratio, whichvaries as a function of b and of α, must be proportional to the linearmagnification of the image to ensure that relief is perceived inrealistic manner by an observer.

EXAMPLE I

Real camera: focal length of real objective lens system to besimulated=200 mm; aperture f/2; image width=57.6 mm, pixel size=0.1 mm;number n of viewpoints=4.

Focusing can begin at twice the focal length, i.e. 400 mm. This leads toimages having the same size in the physical image plane as the filmedobjects. The theoretical stereoscopic field of the apparatus with fourviewpoints extends between 384 mm and 416 mm. An object whose image hasa horizontal dimension equal to one apparent pixel (in fact a lens thatis 0.4 mm wide) forms its image at the focal plane on one lens only, andall four pixels are used. If an object is placed at the limits of thetheoretical stereoscopic depth of field:

at the lower limit (384 mm), the object appears to stand out from thedisplay screen, the image occupies four lenses with a single pixel perlens as follows:

pixel positions in the columns of four: $\begin{matrix}{\left( {1,2,3,4} \right);} & {\left( {1,2,3,4} \right);} & {\left( {1,2,3,4} \right);} & \left( {1,2,3,4} \right) \\{\quad X} & {X\quad} & {\quad X\quad} & {\quad X\quad}\end{matrix}$

at the upper limit (416 mm), the object appears to be behind the screenand the image occupies four lenses with a single pixel per lens asfollows: $\begin{matrix}{\left( {1,2,3,4} \right);} & {\left( {1,2,3,4} \right);} & {\left( {1,2,3,4} \right);} & \left( {1,2,3,4} \right) \\{\quad X\quad} & {\quad X\quad} & {\quad X\quad} & {\quad X\quad}\end{matrix}$

Beyond these theoretical limits, objects form an image spread over morethan four lenses and if a real camera is not stopped down, then pixelsmust share image area with other image points, thus giving anout-of-focus sensation which increases the more the above two limits areexceeded.

The above-mentioned real camera can be simulated by means of fourpinhole cameras with b=3 cm, 2α=4.29°, and an angle θ corresponding tothe field angle of the real camera. The focal length f of the simulatedcameras can be selected in arbitrary manner. The number of simulatedpixels corresponds to the desired resolution, i.e. 143 pixelshorizontally for each simulated camera in order to obtain the resolutionof the real camera.

For synthesized images, lack of focus exists only if it is speciallycomputed. Beyond the above-mentioned limits, and when the observer movesrelative to the display screen, the objects situated beyond theabove-mentioned limits (e.g. 384 mm and 416 mm) move across a portion ofscreen in discrete jumps. So long as the angle 2α does not exceed 4.5°,viewing quality remains compatible with an observer moving relative tothe display screen and with the visual comfort expected of standardvideo. With increasing angle α, at constant stereoscopic base, thefocusing distance decreases. As this distance becomes shorter than thedistance between the observer and the display screen, so thestereoscopic effect increases, with this hyperstereoscopy beingcompensated to some extent by the brain, while the stereoscopic depth offield decreases. There is a limit, corresponding to 2α=4.5°, beyondwhich quality is degraded to such an extent that the observer can nolonger move relative to the screen.

The scene can then be observed only at the “solid color” distance. Thesmall stereoscopic depth of field implies that the objects quicklyproject beyond the volume of the stereoscopic field, therebyconstraining the observer to remain at the “solid color” distance. Whenthe observer moves parallel to the display screen, the apparentmovements of the objects are exaggerated, and can be disagreeable. Formovements of the observer perpendicular to the display screen, thoseportions of an object which project beyond the volume of thestereoscopic field appear as poorly attached surface portions that aretoo big or too small, making observation painful and breaking thesensation of relief for the scene as a whole, thus losing the advantagesof autostereoscopy.

If higher resolution is used and if more than four viewpoints aresimulated, e.g. eight viewpoints, then the object can stay in focuswhile travelling over eight lenses instead of four. If the lenses arethe same size as in the first case, then the stereoscopic depth of fieldis doubled. The necessary horizontal resolution is double using apparentpixels of the same size as in the first case (see below in thedescription corresponding to FIG. 4d).

With changing focus, the angle 2α decreases taking on values that becomesmaller and smaller (asymptotically):

for focusing at 500 mm: 3.43°

600 mm: 2.86°

800 mm: 2.14°

1000 mm: 1.71°

2000 mm: 0.85°

4000 mm: 0.42°

8000 mm: 0.21°, etc. . . .

When the distance doubles, the angle is halved. The ratio of the valueof the stereoscopic depth of field to the value of the focusing distanceis divided by a little more than two, which value increases withincreasing distance. The stereoscopic depth of field becomes veryquickly a significant percentage of the focusing distance. This makes itpossible to determine the second limit value for the angle 2α which itis pointless to exceed, given the available resolution and the number ofviewpoints. In the above example, focusing at 8000 mm gives astereoscopic field that extends from 4444 mm to 40,000 mm for an angle2α of 0.21°. At this distance, the horizon seems somewhat out of focus,but is acceptable, and the sensation of relief beyond the zone ofstereoscopic depth of field is of no advantage using standard videoresolution.

The useful limiting values on the angle 2α therefore lie in the range4.5° to 0.20°.

Although it is not possible to interpret exactly the relationship thatexists between the parameters of human vision in reality and those thatapply to the real autostereoscopic picture-taking system, it is veryeasy to see that the method gives an impression of realism and ofconformity in the images obtained.

This data can be used as being substantially equivalent to data on humanbinocular vision, or it can be considered that it introduces little orno distortion in the perception of relief via interposed video. Frombirth, our visual system has served as a basis for mental imageprocessing which enables us to perceive relief: focal length, fieldangle, effective field angle as perceived through a window placed at acertain distance (equivalent to a television screen of the same apparentsize), inter-pupil distance associated with physiological mechanisms forvergence, focusing, balancing the body, and estimating movement are allspecific to each individual and have an influence on that individual'ssensations.

The set of proprioceptive perceptions (internal perceptions of muscularforces and of ligament tensions required for balance, mobility,binocular fusion, etc. . . . ) constitutes a source of information thatcannot be disconnected; this has consequences that are taken intoaccount very little in conventional systems for taking pictures forshowing images in relief.

The dimensions of an object in the two directions parallel to the planeof the screen (x and y) are divided by two when the object is twice asfar away. It is possible to measure them by projection onto an imageplane. The impression of relief (estimation of the third dimension (z))can only be done by comparison between two flat images forming astereoscopic pair. The difference between them (disparity, or quantityof apparent movement parallel to the image plane of objects in the scenerelative to one another) makes it possible to create an impression thatdepends on the processing performed by the brain. It is very easy tointroduce parameters that fool the observer by deforming estimated depthvalues. By selecting an angle 2α that is constant and by changing thepicture-taking distances at the cost of a proportional modification tothe stereoscopic base and to the focal length (which corresponds to asystem at constant aperture: in our example f/2, and at constant field)it is possible to magnify or shrink all three dimensions uniformly, andfor example a cube continues to be perceived as a cube but larger orsmaller. An object is perceived of size that changes in inverse ratio tothe size it would have had if our eyes were located at the pupil of thesimulated objective lens system.

By modifying the stereoscopic base b without changing the otherparameters, hyper-relief or hypo-relief is created (the object appearsto be squashed or stretched in the depth direction) because the angle 2αis modified without modifying the focusing distance. The magnitudes (xand y) of an object are unchanged (estimation of object size and ofdepth being tied to vanishing traces in monocular vision), whereas depthas estimated by the stereoscopic effect is exaggerated or diminished,given that the disparity is changed.

To conclude, for human vision, when taking pictures or computing images,there exists ideally only one ratio between variation of disparity andvariation of apparent dimensions associated with perspective for a givenfield angle. Changing the field angle (the “zoom” effect, or the“magnifying glass” effect on a portion of the image) is never equivalentto moving the camera towards the object (the “travelling” effect)because although the magnitudes (x and y) in a plane parallel to theimage plane do indeed increase in the same manner in both effects, theratio of the distance between the observer and the planes is changedonly by “travelling”, and so perception of depth is modified in thatcase only. We do not have “multifocal length” vision nor do we have“zoom” vision, and such effects seen in relief always give rise todisturbances to the sensation of depth. Only a “travelling” shot inrelief can match reality exactly, providing the objective lens systemused has a field angle that is equal to that formed by the screen at theobservation distance, assuming the apparent magnification of an objectfilmed at the same distance as the distance between the spectator andthe screen is equal to one.

If linear magnification is not equal to one, then it is necessary tointroduce a correction coefficient to evaluate the field angle of thecamera by considering the size of the screen that would indeed makelinear magnification of one possible if located in the position of thereal screen. A final point is that the focusing distance can bedifferent from the screen observation distance. A collimating effect canbe introduced artificially or in the absence of the small correctionsdescribed above. We are culturally very used to looking at televisionscenes filmed from a distance. The apparent field is greater therein.Consequently, with respect to relief, the effect of disparity is smallerthan it would be in directly observed reality, however monocular cluesenable the observer to transpose something seen with excessive vergenceforce and to compensate somewhat for the loss of sensation of depthbecause of the significant increase in the depth of field of theperceived scene.

The above considerations apply to setting the observed display screen atthe ideal, “solid color” distance. If the observation distance ischanged, by getting closer to the display screen, then to remain underexactly the same conditions, it is necessary to decrease the distance Dof the camera to the object, and the field angle (e.g. horizontal fieldangle) must be increased in order to make the changed field angle formedby the display screen correspond with the new viewing distance. Thestereoscopic base does not change.

If a screen of given dimensions at a given distance is replaced with ascreen that is x times larger, and that is seen from a distance that isx times greater, then the perceived relief is exaggerated, but so longas x remains less than 3 or 4, the sensation of relief is considered asremaining acceptable so long as 2α lies in the above-specified limits(0.20°-4.5°).

EXAMPLE II

Real camera: focal length=50 mm, aperture f/2, image width=576×12.5 p;width of one pixel=12.5μ; four viewpoints. The focal length and thestereoscopic base are one-fourth of the values in Example I. Theapparent magnification is therefore four. The field angle is halved.

Filming must be at one-fourth the distance to have the same angle 2α. Insimulation, the stereoscopic base b is one-fourth its previous value.The disparity is the same, but the perceived size of background objectsis a little large because of the change of field angle (whichcorresponds to a “zoom” effect). This does not matter since we arecapable of correcting mentally so long as the effect is not too great(up to about three times). This evaluation takes account of the factthat we can move closer to the screen without difficulty to see thepairs (1 and 4) or to move away from it to see the pairs (1 and 2) or (2and 3) or (3 and 4), with this giving rise to an apparent change of theangle subtended by the screen by a factor of three, on average. The factof looking at a smaller scene is unusual in relief, and the exactevaluation of depth makes a little more use of our mental compensation.

FIGS. 3a and 3 b show signal processing for going from each of theelementary images 1, 2, 3, and 4 (n=4) given by the cameras C₁, C₂, C₃,and C₄ to an autostereoscopic image that may be displayed on a screen10. Each of the images 1, 2, 3, and 4 is made up of X rows and Ycolumns, with each pixel having anisotropic resolution equal to thenominal resolution in the vertical direction and to 1/nth (i.e. ¼) ofthe nominal video resolution in the horizontal direction.

In FIG. 3a, the image 5 having X rows and nY columns is made upsuccessively of the first column of image 1, the first column of image2, the first column of image 3, the first column of image 4, the secondcolumn of image 1, and so on. The image 5 can be displayed directly on astereoscopic video screen 30 fitted with a lens array.

In FIG. 3b, the images 1, 2, 3, and 4 are placed side by side to form animage 6 having X rows and nY columns comprising four flat images ofanamorphosed format. The first Y columns of the image 6 are occupied bythe pixels of the Y columns of image 1, the following Y columns by the Ycolumns of the image 2, and so on. This thus corresponds to the “nimage” embodiment of patent FR-2 705 006 which is particularly suitablefor recording and/or transmission. For example, in FIG. 3b, atransmitter 21 transmits the images 6 which are received by a receiver22 and the columns thereof are permutated by a decoder 23 to obtain animage such as the image 5 displayable on the stereoscopic video screen20 fitted with a lens array.

FIGS. 4a to 4 d show four particularly advantageously variants of theinvention.

In FIG. 4a, starting from a four viewpoint system fitted on the screen10 or 20 having a lens array 30 of given pitch, it is possible toimprove the fineness of the image by increasing the number of pixels inthe horizontal direction on the simulated camera and on the screen 10 or20. A finer lens array 31 is then used on the screen 10 or 20. Thiscorresponds to a real camera having a lens array of smaller pitch, andfor which it is necessary to stop the sub-pupils down slightly in orderto conserve the depth of field in the picture. In simulation, nothingchanges and the stereoscopic depth of field is divided by two, but sincethe pitch of the lens array is twice as fine, the overall sensation(perception of apparent continuity) is the same.

In FIG. 4b, the area of the image can be increased by adding pixels andcylindrical lenses. The screen 10 or 20 is increased in proportion andthe lens array 30 is replaced by a larger array, but having the samepitch and the same focal length. This corresponds, for example, tochanging over to 16/9 format, or cinemascope format. The parameters ofthe picture-taking system do not change and the stereoscopic depth offield remains the same.

In FIG. 4c, it is possible to increase the solid angle of perception byincreasing the number n of viewpoints without changing the angle 2αbetween two contiguous simulated cameras. No change is made to thenumber or the size of the cylindrical lenses, however the size of theimage points (pixels) is decreased as is the focal length of thecylindrical lenses in the lens array 33 compared with that of the array30. With a real camera, this corresponds to a larger pupil at equalfocal length and to a decrease in the focal length of the lens array incorrespondence with the increase in aperture angle, and also a decreasein pixel size. As before, the stereoscopic depth of field remainsunchanged.

In FIG. 4d, angular resolution is increased in the observation solidangle by decreasing the angle 2α between two adjacent viewpoints inproportion to the number of additional viewpoints that are added. Thearray 30 is conserved and only the number of pixels is changed whentaking pictures and when reproducing them. The result is an increase inthe stereoscopic depth of field in proportion to the number ofadditional viewpoints since, to reach the limits of the stereoscopicdepth of field, the image of an object moving from the sharp point alongthe bisector of the optical axes of the two extreme simulated cameras C₁and C₄ must travel through n lenses (i.e. n² pixels), and the size ofthe lenses does not change, together with an increase of resolution inthe depth direction. This case is particularly advantageous when thenumber n of viewpoints exceeds 4. The angle 2α then lies in the range18°/n to 0.8°/n. For example, for 6 viewpoints, 2α varies over the range3° to 0.13°.

There are two different ways of creating converging optical axes for then cameras (with n≧3).

The first way consists in positioning them as shown in FIGS. 1 and 2a.

In the nominal position, the simulated image planes or screens E₁ to E₄are tangential to a circle of radius equal to the simulated focusingdistance (O₁O=O₂P=O₃P=O₄P=D′). The optical axis (A₁, . . . , A₄) of eachsimulated camera, defined as the axis perpendicular to the plane of itsscreen (E₁, . . . , E₄) and passing through the point P₀, also passesthrough the corresponding pin-hole (O₁, . . . , O₄). This system suffersfrom a relative drawback when the simulated focal length is short (largefield) and the focusing distance is close up. The rotation of the imageplanes causes objects that are far from the horizon and from thevertical axis passing through the center images to have different sizes,thereby limiting the capacity for binocular fusion because correspondingpixels from two viewpoints have different heights. Rotation of thecameras gives rise to a rectangle (window equivalent to screen E₁ to E₄)of images deformed into trapezoids in the vertical direction.

The four focus planes passing through the point P are not coplanar,given that the focus planes are situated in the image planes.

The second solution is the preferred solution and it is shown in FIG. 6.It consists in placing the n cameras with their optical centers O₁, O₂,O₃, and O₄ in lignment on an axis x′x and with their simulated ensitivesurfaces or screens E₁ to E₄ mutually parallel, i.e. with their opticalaxes A₁ to A₄ parallel to one another and to the axis x′x, and insliding the four simulated optical systems laterally so as to beoff-center in the directions of the arrows so as to align the respectivecenters P₀ of the simulated images with the respective optical centers(O₁, . . . , O₄) of the corresponding optical systems and with the sharppoint P. For a given rectangular image, its center P₀ is the point ofintersection of the diagonals of the image framed for display purposes.To a first approximation, it is assumed that the point P is at the samedistance D′ from the optical centers of the simulated cameras, i.e.O₁P≈O₂P≈O₃P≈O₄P. In other words, the points of intersection P₀ betweenthe straight lines O₁P, O₂P, O₃P, and O₄P with the simulated imageplanes E₁, E₂, E₃, and E₄, determine the nominal centers of the imagesto be displayed on the projection screen or back-projection screen.Under such circumstances, a rectangle equivalent to the screen window(E₁, . . . , E₄) of width e and corresponding to the sharp point P isperceived as a rectangle for each simulated camera. Variation in theinter-camera distance b is performed by modifying the degree ofoff-centering (d₁, . . . , d₄) of the simulated cameras. In FIG. 6, theangles and the off-centering are exaggerated in order to facilitateunderstanding.

The simulated cameras are preferably pinhole cameras.

The other image planes remain mutually parallel, and at infinity thereis no distortion between the four images of a given object. The axespassing via the point P and the optical centers (O₁, O₂, O₃, O₄) of thesimulated cameras are then, in general, no longer the optical axes ofthe simulated cameras. Both solutions lead to the same limiting valuesfor the angles 2α. The inter-camera distance b remains equal to thedistance between the optical centers, e.g. O₁ and O₂ of two adjacentsimulated cameras.

In practice, it is possible to avoid modifying the software used bydefining screen windows E′, i.e. (E′₁, . . . , E′₄) each having a centerP′₀ and a width e′ that is greater than the width e of the simulatedimage and that has the same height (see FIG. 2b). The point ofintersection between the straight line (PO₁, . . . , PO₄) and thecorresponding screen window (E′₁, . . . , E′₄) is the point P₀. Asubwindow (E₁, . . . , E₄) of width e having the desired image formatand whose center is P₀ is extracted from the above-defined screen window(E′₁, . . . , E₄). The program is used to compute the pixels of thesubwindow (E₁, . . . , E₄). There is no point in computing all of thepixels in each of the windows (E′₁, . . . , E′₄).

What is claimed is:
 1. A method of acquiring simulated autostereoscopicvideo images of a scene to be viewed, comprising: starting from storeddata including three-dimensional information about an object or thescene to be viewed on a display screen, implementing n simulatedcameras, wherein n≧3, each generating an image of said scene, and eachhaving an optical center and a simulated sensitive surface having acenter, the simulated cameras being equidistant and spaced-apart by thesame inter-camera distance b which remains constant while a picture isbeing taken with a constant field angle, and wherein each of thesimulated cameras has an axis passing through the center of itssimulated sensitive surface, through its optical center, and alsothrough a point P referred to as the simulated sharp point, situatedsubstantially at the same distance D′ from all of said simulatedcameras, and further comprising generating said scene to be viewed ashaving a nearest point P_(p) and a farthest point P_(e), and wherein theinter-camera distance b and the distance D_(min) between the simulatedset of cameras and the nearest point P_(p) are selected in such a mannerthat for taking said picture and for a sharp point that varies betweenthe nearest point P_(p) and the farthest point P_(e), the angle 2αbetween said axes of two adjacent simulated cameras varies between avalue that is not greater than 4.5° for the nearest point P_(p) of thegenerated scene and not less than 0.2° for the farthest point P_(e) ofthe generated scene, said generated scene comprising only pointssituated between said nearest point P_(p) and said farthest point P_(e).2. A method according to claim 1, wherein for a scene including a pointP_(e) situated at infinity, the inter-camera distance b is selected sothat for the angle 2α having a value of 0.2°, the simulated sharp pointP is situated at a distance D_(max) such that the image of an objectmoving from the distance D_(max) to infinity along the bisector of saidaxes of two extreme simulated cameras moves on the display screenthrough a distance no greater than n² times the pixel pitch.
 3. A methodof acquiring simulated stereoscopic images of an object or a scene to beviewed, comprising: starting from stored data includingthree-dimensional information about the object or the scene to be viewedon a display screen, implementing n simulated cameras, wherein n≧4, eachgenerating an image of said scene, and each having an optical center anda simulated sensitive surface having a center, the simulated camerasbeing equidistant and spaced-apart by the same inter-camera distance bwhich remains constant while a picture is being taken, and wherein eachof the simulated cameras has an axis passing through the center of itssimulated sensitive surface, through its optical center, and alsothrough a point P referred to as the simulated sharp point, situatedsubstantially at the same distance D′ from all of said simulatedcameras, and further comprising generating said scene to be viewed ashaving a nearest point P_(p) and a farthest point P_(e), and wherein theinter camera distance b and the distance D_(min) between the simulatedset of cameras and the nearest point P_(p) are selected in such a mannerthat for taking said picture and for a sharp point that varies betweenthe nearest point P_(p) and the farthest point P_(e), the angle 2αbetween said axes of two adjacent simulated cameras varies between avalue that is not greater than 18°/n for the nearest point P_(p) and notless than 0.8°/n for the farthest point P_(e) of the generated scene,said generated scene comprising only points situated between saidnearest point P_(p) and said farthest point P_(e).
 4. A method accordingto claim 3, wherein, for a scene including a point P_(e) situated atinfinity, the inter-camera distance b is selected so that for the angle2α having a value of 0.8°/n, the simulated sharp point P is situated ata distance D_(max) such that the image of an object moving from thedistance D_(max) to infinity along the bisector of said axes of twoextreme simulated cameras moves on the display screen through a distanceno greater than n² times the pixel pitch.
 5. A method according to claim1, wherein said axes are the optical axes of the simulated cameras.
 6. Amethod according to claim 1, wherein the simulated cameras have theirsimulated sensitive surfaces parallel to one another and disposedsubstantially in a common plane, and wherein the stereoscopic base b isobtained by off-centering.
 7. A method according to claim 1, wherein thesimulated cameras are of the pinhole type.
 8. A method according toclaim 1, wherein each image point of the simulated camera is associatedwith vertical resolution equal to the nominal vertical resolution of thevideo image and horizontal resolution equal to 1/nth the nominalhorizontal resolution of the video image.
 9. A method according to claim1, including a step of interleaving P columns of n elementary images toobtain an interleaved autostereoscopic video image having n×p columns.10. A method according to claim 1, including a step of changing theapparent size of an object or of a scene without disturbing its shapestarting from focusing at said distance D′ by displacing the simulatedcameras without changing the sharp point, the angle 2α being keptconstant, and the value of the inter-camera distance b being changed inproportion to the change in the focusing distance.
 11. A methodaccording to claim 1, wherein the image includes stereoscopic pairshaving a given stereoscopic base B, the viewpoints of said stereoscopicpair being separated by m intermediate viewpoints, where m is an integer≧1.
 12. A method according to claim 11, including a step of viewingunder conditions where, at the ideal, “solid color” distance, anobserver sees one of said stereoscopic pairs having viewpoints that areseparated by m elementary viewpoints.
 13. An autostereoscopic videosystem comprising: apparatus for acquiring simulated autostereoscopicvideo images of a scene to be viewed, the apparatus comprising adatabase containing stored data including three-dimensional informationabout an object or the scene to be viewed on a display screen, apparatusfor generating n simulated cameras, where n≧3, each generating an imageof said scene and each having an optical center and a simulatedsensitive surface having a center, the simulated cameras beingequidistant and spaced-apart by a common inter-camera distance b whichremains constant while taking pictures with a constant field angle, andeach of the simulated cameras has an axis passing through the center ofits simulated sensitive surface, through its optical center, and througha point P referred to as the “simulated sharp point” situatedsubstantially at the same distance D′ from all of said simulatedcameras, the scene to be viewed having a nearest point P_(p) and afarthest point P_(e), said apparatus for generating n simulated camerasbeing set up so that the inter-camera distance b and the distanceD_(min) between the set of simulated cameras and the nearest point P_(p)satisfies the condition whereby, for said picture and for focusingvarying between the nearest point P_(p) and the farthest point P_(e),the angle 2α between said axes of two adjacent simulated cameras variesbetween a value not greater than 4.5° for the point P_(p) and a valuenot less than 0.2°for the point P_(e); and display apparatus in which anobserver at the ideal, “solid color” distance sees a stereoscopic paircomprising two viewpoints separated by m intermediate viewpoints where mis greater than or equal to
 1. 14. A system according to claim 13,wherein the apparatus for generating n simulated camera is set up sothat, for a scene having a point P_(e) situated at infinity, theinter-camera distance b is such that for the angle 2α having a valueequal to 0.2°, the simulated sharp point P is situated at a distanceD_(max) such that the image of an object from the distance D_(max) toinfinity along the bisector of said axes of the two extreme simulatedcameras moves on the display screen through a distance no greater thann² times the pixel pitch.
 15. An autostereoscopic video systemcomprising: apparatus for acquiring simulated stereoscopic video imagesof an object or of a scene to be viewed, the apparatus comprising adatabase containing stored data including three-dimensional informationabout the object or the scene to be viewed on a display screen,apparatus for generating n simulated cameras, where n>4, each generatingan image of said scene and each having an optical center and a simulatedsensitive surface having a center, the simulated cameras beingequidistant and spaced-apart by a common inter-camera distance b whichremains constant while taking pictures, and each of the simulatedcameras has an axis passing through the center of its simulatedsensitive surface, through its optical center, and through a point Preferred to as the “simulated sharp point” situated substantially at thesame distance D′ from all of said simulated cameras, the scene to beviewed having a nearest point P_(p) and a farthest point P_(e), saidapparatus for generating n simulated cameras being set up so that theinter-camera distance b and the distance D_(min) between the set ofsimulated cameras and the nearest point P_(p) satisfies the conditionwhereby, for said picture and for focusing varying between the nearestpoint P_(p) and the farthest point P_(e), the angle 2α between said axesof two adjacent simulated cameras varies between a value not greaterthan 18°/n for the point P_(p) and a value not less than 0.8°/n for thepoint P_(e); and display apparatus in which an observer at the ideal,“solid color” distance sees a stereoscopic pair comprising twoviewpoints separated by m intermediate viewpoints where m is greaterthan or equal to
 1. 16. A system according to claim 15, wherein theapparatus for generating n simulated camera is set up so that, for ascene having a point P_(e) situated at infinity, the inter-cameradistance b is such that for the angle 2α having a value equal to 0.8°/n,the simulated sharp point P is situated at a distance D_(max) such thatthe image of an object from the distance D_(max) to infinity along thebisector of said axes of the two extreme simulated cameras moves on thedisplay screen through a distance no greater than n² times the pixelpitch.
 17. A system according to claim 13, wherein said axes are theoptical axes of the simulated cameras.
 18. A system according to claim13, wherein the simulated cameras have their simulated sensitivesurfaces parallel to one another and disposed substantially in a commonplane, and wherein the system includes apparatus for off-centering thesimulated cameras.